More to gears than meets the eye

Before the advent of variable speed drives (VSDs), controlling a systems output speed was achieved using gears. The final output speed of a typical squirrel-cage motor running at 1440rpm can be reduced by varying the gear reduction ratio, allowing the same motor to be used for different speeds without a VSD.

Now you can control motor speeds using VSDs, however the drive cannot replace the gear’s other key benefits, torque multiplication and inertia matching, allowing a relatively small, low power, motor to move and accurately control a large load, therefore reducing running costs.

Spur gear

Ask anyone to draw a picture of a gear and you'll get a spur gear, a disk with teeth projecting radially. Used in everything from washing machines to industrial cutting machines, spur gears are cheap and easy to install. They offer good power transmission efficiency and a constant velocity ratio, with the ability to transmit a large amount of power – up to 50,000kW.

However, there are some considerations. Generally, these gears have significant backlash, making them unsuitable for high accuracy. They can also be noisy at high speed and while they have the ability to be variably configured they occupy a large footprint, especially in high ratios as each gear shaft needs to be supported in its own bearings.

Worm gear

So called due to its earthworm like movement, the worm drive consists of two parts, the screw shaped worm gear and the larger spur-shaped worm wheel. Meshed perpendicular to the axis of rotation, the worm gear offers a compact solution and a large single-stage gear reduction can be achieved, although larger ratios suffer from low efficiency. With some modifications, this gear type can offer relatively good precision. 

Planetary gears

Planetary gears are mounted in a way that a numbers of gears, typically three to five, rotate like planets around a central sun gear, surrounded by an outer annular ring gear.

Planetary gears provide a high power density, over 95% efficiency and are very compact. Accuracy is high, with backlash achievable down to 1 arcmin. High reduction ratios can be achieved with the maximum single stage ratio typically being 10:1. Planetary gears are generally more expensive and can require more maintenance.

Strainwave gear

Offering a high degree of accuracy and quality is a strainwave gear, known generically as a harmonic drive. Applications can be demanding, from broadcast motion control, oil and gas extraction, to robotics, aerospace and high precision industrial machine tools.

It consists of three parts. An outer Circular Spline, a fixed ring with gear teeth on the inside, is meshed to an inner Flexspline, a flexible ring with gear teeth on the outside, the Flexspline is meshed together with an elliptical centrally mounted Wave Generator attached to an input shaft.

A strainwave gear offers very high single stage ratios, from 30:1 up to 320:1, in the same space that a planetary gear can only achieve a 10:1 ratio. Despite this, it maintains compact size, very low weight, zero backlash, low component count and very high torque levels. 

The central shaft can even be bored to provide the largest hollowshaft possible on a concentric gear. It's these characteristics that resulted in our Harmonic Drive being chosen for inclusion by NASA on the Mars Rover.

It's clear that the world of gears is more complex than first meets the eye. Making the right choice of transmission for your given application can drastically alter operational efficiency, energy usage and ultimately the total cost of ownership. This is becoming an increasingly important aspect of the decision making process as we move towards cost saving oriented high-accuracy applications.